Width control



B. J. SMITH WIDTH CONTROL March 27, 1962 V SQgeStS-Sheet l INVENTOR BERNARD J. SMITH,

HIS ATTORNEY.

Filed July 20, 1959 March 27, 1962 B. J. SMITH 3,027,500

WIDTH CONTROL Filed July 20, 1959 2 Sheets-Sheet 2 INVENTOR BERNARD J. SMITH,

B'Y @mun HIS ATTORNEY.

United States Patent ()fiice 3,027,500 WIDTH CONTROL Bernard J. Smith, Syracuse, N.Y., assignor to General Electric Company, a corporation of New York Filed July 20, 1959, Ser. No. 828,325 3 Claims. (Cl. 317-200) This invention relates to a width control for use in television receivers in which the scanning of the cathode ray beam in the picture tube is controlled by a magnetic deflection yoke.

Whereas the vertical dimension of the image can be controlled by the simple expedient of connecting a potentiometer in the circuit of the vertical oscillator, it has been customary to control the horizontal dimension by inserting a rather expensive variable inductor in the horizontal deflection circuit.

'As the deflection angle of the cathode ray tubes has been increased, the diameter of the portion inside the deflection yoke has been decreased to such a degree to conserve power that the electron beam often grazes the inner surface of the tube Wall causing emission of electrons therefrom that eventually reach the phosphor screen in a random manner so as to reduce the contrast ratio of the image.

One of the limitations of the shortness of the cathode ray tube is the proximity of the yoke to the focusing electrode of the electron gun because the magnetic fields of the yoke will cause a marked defocussing of the beam and a consequent loss of detail in the image.

It is an object of this invention to provide an inexpensive means for controlling the width of the area scanned by the electron beam of a cathode ray tube in response to the magnetic fields produced by a deflection oke.

y It is another object of this invention to provide a width control for an electromagnetically deflected electron beam that improves the contrast of the images formed by the beam in cathode ray tubes where the beam tends to graze the inner surface of the tube.

It is still another object of this invention to provide a width control for an electromagnetically deflected electron beam of a cathode ray tube that also reduces the deleterious effect of the electromagnetic field of the focus of the beam.

Briefly these objectives may be attained in accordance with the principles of this invention by mounting tabs of highly permeable material close to the rear of the horizontaldeflection coils of the deflection yoke in such manner that they can be adjusted in a direction perpendicular to the axis of the tube.

The manner in which this is done will be more apparent after the following discussion of the drawings in which:

FIGURE 1 shows a single deflection coil of saddle configuration.

FIGURE 2 is a view of a magnetic deflection yoke mounted on the neck of a cathode ray tube having the tabs'of the invention incorporated therein;

FIGURE 3 is an axial view from the rear of the deflection yoke taken at section AA; and showing the tabs in a position to reduce the width of the area scanned;

FIGURE 4 is similar to FIGURE 2 except that it shows the tabs in a position that permits an increased width of the area scanned;

FIGURE 5 is a cross sectional view in a plane through the axis of the yoke showing certain details of the structure for retaining the tabs in position.

FIGURE 6 shows the detail of a frame also shown in FIGURE 4 for retaining the tabs in position; and

FIGURE 7 is a graph of the horizontal deflecting field along the axis of the yoke when the tabs are in one extreme position and when they are in the other extreme position.

The invention will be described in connection with a particular design of a type of a magnetic deflection yoke that is in common use in television receivers, but it should be understood that it can be used with other types of yoke.

As is well understood by those skilled in the art, nearly all the yokes used in television receivers are comprised of four coils of saddle configuration such as shown in FIGURE 1. Each coil has end turns 1 that are bent outward at about from the side turns 2. One coil 3 is mounted on top of the neck of a cathode ray tube (not shown) and another coil 4 is mounted on the bottom. Circuits are provided for producing current in these coils that varies from a maximum value in one direction to a maximum value in the other direction so as to produce in the volume between the coils a vertical magnetic field parallel to the axis 6 of the coils. In accordance with well known laws of physics such a vertical magnetic field deflects a beam of electrons directed along'the axis 8 of the cathode ray tube (not shown) in a horizontal direction. In a television receiver constructed so as to operate in response to signals presently approved by the F.C.C., the beam scans a series of parallel horizontal lines at the rate of 15,750 a second. Accordingly, means are provided for producing currents in the horizontal windings that vary from maximum to minimum value at the same rate.

The other two coils, not shown in FIGURE 1, are similar to the coils 3 and 4 and are mounted on opposite sides of the neck of the cathode ray tube. Circuits are provided for causing current to flow through them that varies from a maximum value in one direction to a maximum value in the other at a frequency of 60 times a second. The horizontal magnetic field produced thereby causes the beam to move downward so that each horizontal line is vertically displaced from the preceding one.

The coils may be held in position in various ways but a common method is illustrated in FIGURES 2, 3, 4 and 5. In FIGURE 2 there is shown the bulbous portion 10 of a cathode ray tube that adjoins the neck portion 12. A coil form having a forward flange 12 and a rear hub 14 is mounted over the neck 12 and part of the bulbous portion 10.

the side turns being blocked from view by an annular collar of highly permeable magnetic material. In this view the horizontal deflection coils are not visible because they are mounted, in this particular yoke design, be

tween the coil form and the cathode ray tube. In FIG- URES 3, 4 and 5 the rear end turns 1 of the horizontal coils are visible, and in FIGURE 5 the side turns 2 can "be seen.

Turning now to FIGURE 5, which is a vertical cross section through the axis 18 of the cathode ray tube, it

will be observed that the end turns 1 of the horizontal deflection coils extend radially into the top and bottom of the hub 14 of the coil form. Slits 20, only one of which appears in FIGURE 5, are provided on opposite sides of the hub 14. A frame 22, shown in detail in FIGURE 6, fits in the slits 20 and provides a means are sufliciently resilient, they may be made to retain any adjusted position by making them slightly wider than Patented Mar. 27, 1962 In this particular view, only the end turns 1 of the vertical deflection coils are visible,

Zl and 26' are not presentr the frame 34 so that when bowed so as to fit in the frame they will press against it when released, On the other hand, if the tabs 24, 26 are too rigid to function in this manner, the necessary resiliency can be provided by placing a wavy piece of resilient material 36 between one side of the frame 34 and the tabs 24, 26.

The manner in wh-ich the tabs 24, 26 operate to adjust the width of the area scanned by the beam will now bedexplai nedwith the aid of FIGURES 3, 4, and '7.

Although most of the vertical magnetic flux produced by the side turns 2 of the horizontal deflection coils lies between the end turns 1, it is well known that in air a discontinuity in the density of the flux cannot exist so that the lines of flux are close together at one point and nonexistent at an adjacent point. Accordingly, if the i ta-bsare not present, the vertical flux extends rearwardly as indicatedby the dotted lines- 40 in FIGURE 5 and causes the deflection of the beam of electron to start before the electrons reach the rear endturns ljof the horizontal deflection coils 3 and; 4.- The s lid'line..,42,

the center of deflection and greatly reduces the tendency of the beam to graze the inner surface of'the neck 12 when a raster is being swept;

There is a trend toward making cathode ray tubes of! less depth which results in bringing the electron guns close to the rear of the deflection yoke. For reasons understood by-those skilled in the art, the magnetic flu defocuses the electron beam, and because the tabs 24-; 2

V of this invention reduce this flux, they improve the focus.

of FIGURE ,7 illustrates in general the, density of-thev vertical lines of flux, or in other words-the field. strength along the axis 18, the approximate location o-fthe, end

turns lbeingindicated byj thevertical lines 44" and "46.

When properly adjusted, tabs 24 and 2 6 reduce thef-flu'x, to their rear with the result that the field strength along. the axis1 8 i-sgenerally as indicated by the dotied, line 52 of FIGUREJ. "The angle of deflection is proportional to the area under a curve representing the flux directly along the axis-lfi and it can readily be seen that the area under the curve 52 islessthan ,that-under'the curve 42 by an amount equal to the shaded portion so that the beam is deflected through a wider' angle when the tabs The following discuss on explainswhy the curves 42 and 52' are differentq with the tabs 24 and 26 widely separated; as in FIGURE 4 little-change is produced in deflectioncoils, said secondpairof deflection coils being shorter than said first pair, means, for mounting saidfirst pairfiof coils with their axes coinciding and at right angles tq theyaxes ofqsaid second pair, said axes-of saidfirst ,pair and said of; said second pair being perpendicu lar' toj a, reference axis of' said yoke, awidth controi corh'prising' first; and second tabs of highly permeable magneticmaterial, means for mounting said tabs to the rear of said first pair of magnetic deflection coils to) perJ-r the distributionof the vertical flux, and,- as shown by the arrows 48 the flux paths to the rear of the end turns 1 extend in substantially straight lines; across the neck 12. When the tabs 24 and 26 are moyed inwardly, as

illustrated in FIGURE 3, the flux'isbent away from the axis 30 asindicated by the curvedar'r ows 50, and the flux distribution along the axis 18 is as indicated by the dotted line 52, Bending of the flux could be produced by a thin ringof highly permeable .rna-terial, but-this would not prevent the flux flowing outside the ring from creating afairly stro'ng field at some point on the axis to the rear of the ring. The tabs 24, 26 extend on either side of the neck 12 to a point where the vertical flux extending to the rear of the yoke produces a negligible field at the axis 18. The effect of the tabs on the width of scanning varies inaccordance with the position of the tabs, the

, position shown in FIGURESBand 4 being extreme-s.

,The vertical deflection coils also produce flux to the rear of their end turns 1, but the density of the fluxand hence the field produced at -apoint where the, axis 18 intersects the plane of the tabs 24, 25 and rearward thereof, is small in comparison with the vertical flux density. Whatever small effect the tabs 24 and 26 have on the vertical dimension of the area scanned by the electron beam is in the same sense as theireffect on the ,7 horizontal dimension,

Thernaterial used for thetabs must have low losses so as, th avoid an-undue reduction in the Q of the horizontal defleetionsystemt Satisfactory results have been attained with tabs of Sinimax having a thickness of A yoke assembly-using the tabs of this invention improves the picture contrast ratio and the-focusof the.

beam of electrons. As can be seen from FIGURE 7,

when thetabs. 24, 26 are moved inwardly, thecentr'oid.

J pair of elongated tabs of thin highly permeable magnetic mit said tabs to be moved along a line perpendicular to said reference axis so as to provide a low. reluctance" path for the portion of the magneticflux produced by' said coils that extends rearward thereof.

-2. A magnetic deflection yokecomprising a'first pair :of saddle type coils,- means for mounting said first pair ofcoils indiametrically opposed relationshipso that their axes substantially coincide and theirendturns extend in opposite radial directions, a second pair of saddle type deflection coils, means for mounting said second pair ,of deflection coils so that their axes substantially coincide and are substantially perpendicular to the axes of said first pair, saidsecond'pair of coils being shorter than said first; pair so that their end turns are closer to theaxes of the; two pairs of coils, a widthcontrol comprising a? material, means for mounting'said tabs in'a planeparallel to saidiaxes of said coils near the end turns of said.

first ,pair of coils, said mounting beingsuch as to permit said tabs to .be slidalong a line, in said latterplane that "is substantially perpendicular to the axes of said first pair of coils.

13. In a-rnagneticdeflection yoke for a television receiver having first and second pairs of diametrically opposed deflection coils mounted around an axis, so as to. produce, when energized, magneticflux in-mutually perpendicular directions each of vwhich is perpendicular to. said axis, means for controlling the maximum deflec-' 'tion affordeda beam of electrons passing throughsaid magnetic flux comprising a pair of tabs ofhighly permeable magnetic material, means for mounting said tabs of the area is irnoved forward i-.e. the centroid under the 1 curve52 is further forward than the centroid of the area under the curve 42-. Thi roduces. a forward shift in for movement in a plane, said plane being perpendicular,

to, said axis and-adjacenttothe ends of said coils, said mounting means-also;restricting saidmovement along, a line insaid plane that is perpendicular to themagnetic flux; produced by one of said-coils and parallelltothe magnetic flux produced by the other.

References Cited in the file of this patent V UNITED STATES PATENTS Haugen c. a July 8, 1952' i 2,704,816 Ferns'ler Mar. 22, 1955 2,793,311 Thomas May 2-1, 1957' 2,861,209 Biggs Nov, 18, 1958 2,899,578 Kirkham Aug. 11', 1959 2,901,665 Barkow 1V t Aug. 25, 1959 

